A big part of what we do is invite people to be neighborly. This can be as simple as waving to your neighbors, chatting with folks for a few minutes in your front yard, and sharing stuff like tools and food. Honestly, it is pretty small stuff...but it has a surprisingly big impact. I recently read this research:

A 2006 study (Tomaka) showed a positive association between ‘belongingness support’ (which includes talking daily with friends or neighbors) and health outcomes. While another study from 2007 (Wilson) showed the risk of Alzheimer disease is twice as high in older people who are lonely as it is in those who are not. 

Isn't that crazy? I'll be keeping this in mind the next time I'm leaving home and have a chance to visit with one of my neighbors for a few minutes.